The present invention relates to a scanning electron microscope for scanning an electron beam on the surface of an observation sample such as an IC, detecting a secondary signal generated from the sample, whereby obtaining a two-dimensional scanned image indicating the shape or composition of the sample surface.
The scanning electron microscope accelerates electrons emitted from a heating or field emission type electron source, forms a fine electron beam (primary electron beam) using an electrostatic field lens or a magnetic field lens, scans the primary electron beam two-dimensionally on a sample to be observed, detects a secondary signal, such as secondary electrons or reflected electrons, generated secondarily from the sample by irradiation of the primary electron beam, and converts the intensity of the detected signal to an brightness modulation input of the CRT scanned in synchronization with scanning of the primary electron beam, whereby obtains a two-dimensional scanned image.
A general scanning electron microscope accelerates electrons emitted from an electron source with a negative voltage applied between the electron source and the anode at the grounding voltage and scans the primary electron beam on a test sample at the grounding voltage.
When observing the processing shape of a wafer in the semiconductor process by the scanning electron microscope, to prevent the insulator within the wafer from charging by electron scanning, the shape is observed at a low acceleration voltage of 2 kV or less. This relates to the secondary electron generation efficiency δ generated when electrons are irradiated to a substance. In this case, the secondary electron generation efficiency δ is defined by [(secondary electron amount)/(primary electron amount)].
FIG. 1 shows the relationship between the secondary electron generation efficiency δ and the acceleration voltage. When the acceleration voltage (1 kV to 2 kV) when the secondary electron generation efficiency δ becomes almost 1 is selected, electrons entering a sample (incident electrons) and electrons (secondary electrons) coming out from the sample are equal in number and hence generation of charging can be prevented. The acceleration voltage when the secondary electron generation efficiency δ becomes 1.0 is almost 1 to 2 kV though it varies with a substance. At the acceleration voltage when the secondary electron generation efficiency δ is more than 1, discharge of secondary electrons is stronger than incidence of primary electrons, so that the surface of the insulator is positively charged. This positively charged voltage is several volts at most and stable, so that observation of scanned images provides no trouble. However, within the range from 1 kV to 2 kV, the secondary electron generation efficiency δ may not be 1 or more depending on a sample. As a result, an unstable negative charge is generated. Therefore, in the case of observation of a wafer including an insulator by a conventional scanning electron microscope, an acceleration voltage within the range from 500 V to 1000 V which allows the secondary electron generation efficiency δ to exceed 1.0 and sufficiently accelerates the electron beam is selected.
A semiconductor wafer is observed under such a condition, though a big problem in terms of practical use is observation of a deep contact hole.
A contact hole 102 is used to electrically connect a conductive board 103 to wires (not shown in the drawing) formed on the top of an insulator 101. The object of observation of the contact hole is to check the opening of the hole 102 for etching the insulator 101. Unless the conductive board 103 is exposed surely on the bottom of the contact hole 102, even if the contact hole 102 is filled with a metal (deposition), it is a bad conductor which cannot connect with the conductive board 103.
The contact hole is observed by displaying the condition of the contact hole 102 formed in the insulator 101 provided on the board 103 on the display screen of the scanning electron microscope. As shown in FIG. 14A, when the contact hole 102 perfectly reaches the board 103, the bottom of the contact hole 102 clearly shows the shape of a bottom B and it is observed that the good contact hole 102 is formed. However, as shown in FIG. 14B, when the contact hole 102 does not perfectly reach the board 103 and there are residues on the bottom, the bottom of the contact hole 102 is observed as a shadow 102B.
In such an observation, as shown in FIG. 2, a large part of secondary electrons 104 generated at the bottom of the contact hole 102 collides with the wall of the hole 102 and disappears and only a part of secondary electrons 104a emitted upward gets out of the hole. When the contact hole is shallow (the aspect ratio<1 to 2), although signals are reduced, a considerable part of secondary electrons gets out of the hole 102, so that the hole can be observed. However, when refinement advances like recent semiconductor devices and the aspect ratio is more than 3, it is impossible to observe the bottom of the contact hole. As an example of difficult observation by such a conventional scanning electron microscope as shown in U.S. Pat. No. 5,412,209, for example, a sample example shown in FIG. 3 may be cited.
When a sample that metal wires 105, for example, aluminum wires are embedded in the insulator 101 is observed at a low acceleration voltage causing no charging, as mentioned already, the surface of the insulator is charged positively and stably by balancing of secondary electrons. Therefore, even if the internal wires 105 are provided, the scanning electron microscope cannot observe the existence thereof.